Add cron container and per-household timezone for auto-reminders

- docker-compose.yml: meal-cron service loops send_reminders_cron.py every 60s
- send_reminders_cron.py: checks each household's local time, sends ntfy
  reminders at 10:00 (lunch) and 16:00 (dinner), guarded against duplicates
- models.py: timezone, last_lunch_reminder, last_dinner_reminder on households
- meals.py: /admin/timezone route for admins to set household timezone
- dashboard: timezone selector (20 common zones) in Account Settings
- i18n: 5 new translation keys for timezone strings
- Dockerfile: copy send_reminders_cron.py
- tests: 4 new timezone tests (admin set, non-admin denied, default UTC, auth)
- docs: updated AGENTS.md

i18n.py

@@ -366,6 +366,28 @@ TRANSLATIONS: dict[str, dict[str, str]] = {
         "fr": "Échec de l'envoi du test. Vérifiez votre URL ntfy et votre jeton.",
         "de": "Testbenachrichtigung fehlgeschlagen. Überprüfen Sie Ihre ntfy-URL und Ihr Token.",
     },
+
+    # ── Timezone ──
+    "Household Timezone": {
+        "fr": "Fuseau horaire du foyer",
+        "de": "Zeitzone des Haushalts",
+    },
+    "Used to send reminders at 10:00 (lunch) and 16:00 (dinner) local time.": {
+        "fr": "Utilisé pour envoyer les rappels à 10h00 (déjeuner) et 16h00 (dîner) heure locale.",
+        "de": "Wird verwendet, um Erinnerungen um 10:00 (Mittagessen) und 16:00 (Abendessen) Ortszeit zu senden.",
+    },
+    "Only admins can change the timezone.": {
+        "fr": "Seuls les administrateurs peuvent changer le fuseau horaire.",
+        "de": "Nur Administratoren können die Zeitzone ändern.",
+    },
+    "Please select a timezone.": {
+        "fr": "Veuillez sélectionner un fuseau horaire.",
+        "de": "Bitte wählen Sie eine Zeitzone aus.",
+    },
+    "Timezone updated.": {
+        "fr": "Fuseau horaire mis à jour.",
+        "de": "Zeitzone aktualisiert.",
+    },
 }